The field relates to two-stroke cycle engines, particularly to nonreversing loading of journal bearings in the piston/crankshaft linkages of two-stroke opposed-piston engines.
In the 4-stroke cycle of a conventional crank-rod-slider engine, the inertial force of a piston assembly imparts a negative (i.e. opposite direction) load on a wristpin bearing during the exhaust stroke. During this load reversal period the load bearing surfaces in the wristpin separate and lubricating oil enters into the small gap between the pin and the bearing surface. This oil supply is critical for the bearing to operate in a full hydrodynamic mode, due to squeeze film generation and/or relative motion between the components.
In some aspects of two-stroke cycle engine operation, due to the nature of the cycle, a load reversal on journal bearings may never occur during the normal speed and load range operation of the engine; or, the duration of a load reversal might be relatively short. For example, during operation of a two-cycle diesel engine, a combustion event occurs every cycle and there is nearly always a gas pressure loading on the crown of a piston near top center (TC), which, even at high piston speeds, is still greater than the inertial force of the piston assembly on a crosshead bearing in the piston/crankshaft linkage. At the other end of the cycle, at bottom center (BC) the inertial force of the piston assembly keeps the crosshead bearing loaded as well. As a result, the bearing is nearly always under positive load throughout the cycle. Hence it is difficult to replenish the crosshead bearing with oil. Furthermore, given limited angular oscillation of the bearing, oil introduced between the bearing surfaces does not completely fill the bearing. Eventually the bearing begins to operate in a boundary layer lubrication mode (also called “boundary lubrication mode”), which leads to excess friction, wear, and then bearing failure.
In two-stroke cycle engine construction, nonreversing loading of journal bearings has been addressed in several ways, for example by use of 1.) rolling element bearings, 2.) high pressure lubrication to separate the bearing surfaces and force oil into the joint, 3.) sectored bearings to entrain oil flow in between the surfaces, 4.) reduction of engine load to allow for reduced bearing pressures, or 5.) combinations of some or all of the above items. These measures can address the need for adequate lubrication in engines of moderate load and durability requirements but they are of limited effect for a highly loaded, long life (10,000+ hour) two-stroke cycle engine.
A rocking journal bearing construction as shown in FIG. 1 has been proposed for reducing the nonreversing load problem in journal bearings of two-stroke cycle engines. In the figure, the bearing elements are shown separated with relative dimensions exaggerated in order to more clearly present offsets between segments of the bearing. As per FIG. 1, a rocking journal bearing 10 with multiple offset segments includes a segmented bearing sleeve 12 and a correspondingly-segmented bearing journal 20. The sleeve 12 includes a bearing surface 13 with a plurality of axially-spaced, eccentrically-disposed surface segments, and the journal includes a plurality of axially-spaced, eccentrically-disposed journal segments. The sleeve 12 has a semi-cylindrical configuration with two lateral surface segments 14 sharing a first centerline C1 and a central surface segment 15 separating the two lateral surface segments 14 and having a second centerline C2 offset from the first centerline. The journal 20 has a cylindrical configuration with two lateral journal segments 21 positioned to share the first centerline C1 and a central journal segment separating the two lateral journal segments and positioned to share the second centerline C2. The journal segments are lifted from the surface of the sleeve periodically during a rocking portion of the cycle, thus relieving the load on one or more segments while maintaining the total load on the remaining segments. Separation of the sleeve and journal segments provides clearance for the entry of oil into the joint.
Although journal rocking provides separation allowing introduction of oil between the bearing surfaces, limited angular oscillation of the bearing impairs the formation of a continuous film of oil that fills the volume between the bearing interfaces. During the portion of the bearing cycle when the various journal segments lift, a low pressure region is created on the mating contact region of the sleeve leading to potential voids in the surface film and potential cavitation if not filled adequately. Oil filling thus plays a substantial role in the operation and durability of a rocking journal bearing.
In the prior art, the circumferential borders between adjoining surface and journal segments are grooved to provide for transport of oil around circumferential peripheries of the segments. In certain heavy duty two-stroke cycle engines, axial grooves provided in the surface segments intersect circumferential grooves between the surface segments in order to increase the penetration of oil into the interfaces between surface and journal segments. However, in these bearing constructions, oil is provided via inertia-operated channels that feed into the circumferential grooves of the surface segments, which limits the pressure at which the oil is provided to the bearing interfaces. Moreover, the pressure of the inertia-fed oil fluctuates during engine operation. Consequently, the oil may be fed to the bearing interfaces at pressure levels that are insufficient to maintain continuous oil films of adequate thickness to avoid boundary-layer lubrication, which can result in limited durability and shortened lifetime of these rocking journal bearing constructions.
Adequate penetration of oil into the rocking journal interfaces requires that a continuously-available supply of pressurized oil be timely delivered to a rocking journal bearing for application to the bearing interfaces during a filling cycle.